This application requests support for a 3-year program of research to develop a computer-based simulation model of HIV/STD transmission that can be applied to sexual risk reduction intervention data sets to generate more accurate and informative estimates of the effectiveness and cost-effectiveness of these interventions. Because funds to support HIV/STD prevention interventions is needed to ensure that the most economically-efficient interventions are implemented and that HIV/STD prevention resources are used to maximal benefit. To address this need, our research group and others have initiated research programs to assess the cost-effectiveness of sexual risk reduction interventions using mathematical models of HIV/STD transmission to estimate the number of infections averted by these interventions. These 'deterministic' models are subject to a number of limitations, however. Notably, they do not take into account the relative time of sexual partnerships and sexual acts within those partnerships, and are not easily extended to incorporate time-varying or population-distributed factors that influence HIV/STD transmission. The proposed simulation model improves upon these models by: (1) permitting key parameters and outcomes to vary randomly, both within the modeled population and over time; (2) explicitly modeling the timing of transmission risk activities, thereby allowing for "dynamic" infections that occur when an intervention participant acquires an STD, then transmits infection to one or more partners during the intervention assessment period; (3) simultaneously modeling transmission of multiple STDs (including HIV) and synergistic interactions between STDs; and (4) generating much more informative characterizations of cost effectiveness outcomes in the form of cost-effectiveness ratio distributions and confidence intervals, rather than just the cost-effectiveness point estimates that are produced by deterministic models. The simulation model--which can be applied to a wide variety of sexual risk reduction intervention data sets, study designs, and intervention strategies to estimate HIV/STD intervention cost-effectiveness--will be a valuable tool for intervention researchers, program evaluators, and other interested persons who require a flexible yet standardized model for assessing the cost-effectiveness of HIV/STD prevention interventions.